The invention relates generally to protecting data ports on telecommunications equipment against electrical surges and, more particularly, to providing protection against differential electrical surges.
Electrical surges and electrical surge protection applies to a very broad spectrum of different equipment and environments. This invention will be described for exemplary purposes as being directed to electrical surge protection for LAN data ports of Ethernet networking equipment.
The terms “circuit-side” and “line-side” used throughout the specification and claims identifies which side of the isolation transformer is being referred to. These terms are analogous to the terms “primary” and “secondary” sides of transformers, where primary is considered the input to the transformer and secondary is considered to be the output of the transformer. However, the transformers used in magnetics interface circuits are generally bi-directional and each side of the transformer can be either an input or an output, depending on the circumstances. For the purpose of the specification and claims, the term “circuit-side” refers to the side of the transformer that connects to the equipment's electronic circuitry, whereas the term “line-side” refers to the side of the transformer that connects to the data cables.
10Base-T and 100Base-T Ethernet have 2 channels per port while 1000Base-T Ethernet (Gigabite) has 4 channels per port. For the purpose of explanation, only a single channel of an Ethernet port will be described herein for each embodiment of the invention.
A differential electrical surge event is a very fast high voltage/current spike which gets applied differentially across the input/output (I/O) signal pins by direct contact, air discharge, or induction. The most common causes of electrical surges are lightening, static electricity, and cable discharge. There are a number of other type of electrical surges; however, they will not be further discussed herein as they are not relevant to an understanding of the invention.
An electrical surge applied differentially to the I/O pins of the data port channels will be electromagnetically coupled through the isolation transformer and into the transceiver IC where it can cause catastrophic failure, latent failure or degradation in performance of the networking equipment. This is unlike a common-mode electrical surge; which gets applied to all channels in-common. With respect to ground and will not be electromagnetically couple through the isolation transformer.
To achieve effective differential electrical surge protection, there must be some means to clamp or otherwise suppress the voltage of the electrical surge to a safe level. Because an electrical surge can be a high energy event, the protection mechanism must be capable of handling large amounts of current as well.
The prior art employs bulky and expensive bi-directional semiconductor based voltage clamping devices, commonly called transient voltage suppressors (TVS), to provide protection against differential electrical surge events. These devices are placed on the line-side (RJ45 side) of the LAN magnetics interface circuit, one TVS for each channel of the Ethernet port. The TVS-device clamps the voltage of the electrical surge to a safe level and also shunts the very high surge current of the electrical surge away from the magnetics interface circuit.
FIG. 1 shows a block diagram of a prior art circuit for providing differential electrical surge protection for a single channel within an Ethernet port. As illustrated in FIG. 1, the prior art circuit includes a TVS device 10 and a magnetic interface circuit 14 between the line side 12 and the circuit side 16 of the channel. The magnetic interface 14 circuit can be any number of different circuit topologies. As illustrated in FIG. 2, a conventional magnetic interface circuit 14 includes an auto transformer 18, a common mode choke 20, an isolation transformer 22 and a termination network 24 comprising a resistor 26 and a capacitor 28.
There are many disadvantages associated with the prior art. Most of these disadvantages stem from the fact that the TVS device 10 is required to be placed on the line-side (RJ45-side) of the magnetics interface circuit as shown in FIG. 1. As a result, the following list of problems and disadvantages result;
1. The TVS devices must be capable of withstanding the high energy of an electrical surge event; hence they must be physically large in size.
2. The TVS device is very expensive. Since 10/100Base-T Ethernet requires 2 TVS devices per port and 1000Base-T Ethernet requires 4 TVS devices per port, the cost of the TVS devices alone can exceed the entire material and labor cost of the Integrated Connector Module (ICM) in which the LAN data port is housed.
3. The TVS device of the type used in the prior art has a high capacitance because of its large die size and packaging. This capacitance will degrade return loss, insertion loss and other parameters of the magnetics interface circuit and this will impact signal integrity on a system level.
4. The TVS devices, being located on the non-isolated side of the magnetics interface circuit, are subject to safety agency hi-pot (high-potential) requirements. Hence, all of the TVS components, conductive surfaces and the associated interconnecting wiring must have adequate clearance (spacing) away from any ground referenced conductors. This creates many problems in terms of the physical placement of the TVS devices, the length and routing of the interconnection wiring and/or the layout of the internal PCB (Printed Circuit Board). This can also have adverse affects on other electrical performance issues.
5. The large size of the TVS device also causes a host of other problems:                a. There is very little room available within the ICM to place multiple large TVS devices, particularly because they must be placed close to the RJ contact pins in order to be effective. This usually requires an increase in the size of the ICM or requires larger or additional internal PCBs and/or structures to accommodate the TVS devices, adding significant cost and design complexity.        b. The interconnecting wiring associated with the large TVS devices requires additional space (besides that needed to accommodate the large TVS devices themselves), making wiring and/or PCB layout difficult. Additionally, electrical performance problems may arise if the interconnecting wiring and/or layout of the PCB are not properly done.        c. The large size and limited selection of device packages makes implementation of surge protection into existing designs highly problematic, if not impossible, and also makes the mechanical and electrical design of new products more difficult, which can lengthen the design cycle and time to market.        
6. The circuit configuration of FIG. 1 does not provide protection against common-mode surge events. Hence, additional protection devices are required.
7. The TVS device is connected in parallel with the line-side of the magnetics circuit and, therefore, part of the high current associated with the surge event will flow through the winding(s) of the magnetics. This can cause damage to the wire or open circuits, latent failure, or degradation in performance. The extent to which this is a problem depends heavily on the type of TVS used, the interconnecting wiring, and the design of the magnetics interface circuit.